A young Florida girl is "not the same" after seeing her older sister, 13, shot dead on a school bus early Tuesday, allegedly by a 15-year-old boy.

"She's not right. She's not the same 7-year-old after seeing what she saw," the mother of the two girls told reporters in an interview in English and Spanish. "She was just screaming. So hurt."

The 13-year-old shooting victim, Lourdes Guzman-DeJesus, was sitting a few rows from her 7-year-old sister Tuesday morning in Homestead, Fla., when the 15-year-old shot her, according to the Miami-Dade Police Department.

The 15-year-old is in custody and faces charges of manslaughter and carrying a concealed firearm, though officials told The Associated Press they believe the boy was showing off the gun to the other students and it went off by accident.

Police responded to shots fired inside a private bus around 6:45 a.m., and found several dazed students, including Guzman's little sister, officials said.

"It's traumatic, and now you are going to have to question them and ask them what they saw," Miami-Dade County police detective Alvaro Zabaleta told ABC News. "It's a sad situation."

Lourdes, known to her family and friends as "Jina," was shot an unknown number of times and airlifted to Miami Children's Hospital, where she died from her injuries, police said.

"She was very colorful, smart, nice, such a happy girl," said Guzman's mother, who did not give her name at a news conference but identifies herself on Facebook as Ady DeJesus, according to the AP.

"I loved her so much," DeJesus said. "She was my best friend. She was my partner."

In a written statement obtained by the AP, DeJesus went further.

"Feels like just yesterday I saw her running around in her Pamper, dancing and modeling for the camera," she said. "Times and moments spent with Jina are memories I will cherish and keep in my heart forever."

The boy waived his right to appear in court Wednesday morning and will remain in a juvenile detention center, according to the AP.

"He has to pay for what he did," DeJesus told reporters.

Messages left with juvenile division officials and the alleged shooter's mother were not immediately returned on Wednesday, according to the AP.

The shooting happened just a few minutes away from Palm Glades Preparatory Academy, a charter middle school that the victim attended.

The suspect and the victim's 7-year-old sister attended other charter schools, according to the AP.

A firearm was recovered on the scene and all of the students, plus the bus driver, were transferred to the homicide unit, where Miami-Dade County police began the delicate task of questioning the students about what happened, officials said. The bus did not have a surveillance camera.

Other officers combed the scene for clues and gathered the students' belongings, police said.